1. Field of the Invention
This invention relates to a method of forming a film of ultrafine particles using a high-speed, high-energy beam.
2. Description of the Prior Art
In techniques for forming films of ultrafine particles wherein strong films of metals, ceramics or other ultrafine particles are formed by aerosolizing the particles by gas agitation and accelerating the particles through a fine nozzle as in the gas deposition method (Seiichiro Kashu: Kinzoku [Metal], January 1989, P. 57), or by electrically charging the particles and accelerating them with a magnetic field gradient as in the electrostatic particle coating method (Ikawa, et al., The Japanese Society of Precision Machine Engineering, Fall 1977 Annual Conference Technical Session Preprints, p. 191) and spraying upon and made to collide with a substrate, conventionally, when forming films of functional materials, it is difficult to maintain their structures at the time of the formation of films with ultrafine particles so there were cases in which adequate functionality could not be expressed.
This is because the aforementioned conventional methods of film formation worked on the fundamental principle that a portion of the kinetic energy is converted into thermal energy by colliding with the substrate and thus the ultrafine particles are sintered together and the ultrafine particles are sintered to the substrate. Thus, in the case of oxide materials that have a high melting point, when ultrafine particles are accelerated to speeds of several hundred meters per second or greater in order to obtain a heating temperature sufficient for fusion-bonding, the ultrafine particles are subjected to a large amount of distortion of their crystal structure or are pulverized due to the shock forces occurring at the time of collision with the substrate, so this is a drawback. In addition, this distortion causes a large amount of stress within the film, leading to problems such as degradation of the film characteristics and exfoliation from the substrate. Moreover, if the ultrafine particle material is a metal, an oxide film forms easily on its surface so it is difficult to obtain a film that has sufficient conductivity and adhesion to the substrate. In addition, even in the case of oxide ultrafine particle materials, the adhesion of moisture to its surface or the like reduces bonding among the ultrafine particles so it is difficult to obtain films with good characteristics.
On the other hand, the plasma spraying method is known as a technique of forming films by using a plasma gas to spray particles from a nozzle onto a substrate. This is a technique wherein particles with a particle size of several .mu.m or greater are transported by gas in a high-temperature, high-speed plasma jet generated by ionizing inert gas, and the particles supplied by injection are heated and sprayed and accelerated to collide with the substrate by the high pressure generated at the same time, thus forming a film. The high-temperature plasma jet is obtained by generating an arc discharge by applying a high voltage between the negative and positive electrodes provided within the gun head used for spraying film materials, thus turning gas introduced at nearly atmospheric pressure into a high-temperature plasma.
However, the temperature of the plasma jet thus generated reaches 30000.degree. C. in the hottest areas, so the deposited particles are heated from near the melting point to thereabove thus reaching a semi-molten or molten state and then sprayed at the substrate. Therefore, the crystal structure of the sprayed particles is destroyed and depending on the material, its composition may change due to differences in the vapor pressures of the constituent atoms, and moreover, it is difficult to control the state in which they are cooled and recrystallized upon adhering to the substrate, so there is a problem in which the crystal structure of the deposited film may differ greatly from the crystal structure of the original particle material.
For this reason, in order to give the deposit the crystal structure of the original ultrafine particle material and improve its characteristics, conventionally the deposited film had to be reheated to high temperature either during deposition or after deposition, and this heat treatment became a major problem in the formation of films of functional materials and their fine working in order to form small functional components or device components. Moreover, if the ultrafine particle material is a metal, it is difficult to obtain a film that has sufficient conductivity and adhesion to the substrate.
Moreover, in the case of techniques for the formation of thin films by PVD or CVD without using ultrafine particles, since a growth stage is passed through from the atomic or molecular state, in the case of oxide ceramics materials or the like, high-temperature heat treatment is often needed, and the film formation rate is also more than two orders of magnitude lower than that of the aforementioned film formation step using ultrafine particles, so obtaining a film with a thickness of several .mu.m or greater is difficult in practice.
The invention came about in consideration of the aforementioned conventional drawbacks, and its object is to provide a method of forming films of ultrafine particles whereby even if a stream of ultrafine particles collides with the substrate at low speed, a strong bond is achieved between the ultrafine particles and the substrate at a low-temperature state, so the crystal properties of the ultrafine particles are maintained and a thin film with superior density and excellent adhesion is formed.